In the field, two types of ammunition are generally recognized: traditional supersonic ammunition, which fires projectiles with velocities exceeding the speed of sound; and subsonic ammunition, which fires projectiles with velocities less than that of the speed of sound. This low-speed characteristic of the subsonic ammunition makes it much quieter than typical supersonic ammunition. The speed of sound is variable depending on the altitude and atmospheric conditions, but is generally in the range of 1,000-1,100 feet per second (fps), most commonly given at 1,086 fps at standard atmospheric conditions.
Ideally, these subsonic rounds need to work interchangeably with supersonic rounds in their ability to fit properly in the same firearm chamber. The traditional method of forming subsonic rounds is to simply reduce the propellant charge in the shell until the velocity is adequately reduced. Unfortunately, this solution is not ideal for a number of reasons. Principally these problems are rooted in the relatively large empty volume inside the case left vacant by the reduced propellant charge. This empty volume inhibits proper propellant burn, results in inconsistent propellant positioning, causes reduced accuracy, and, in special situations, may lead to extremely high propellant burn rates or even propellant detonation, an extremely dangerous situation for the weapon user. For example, since the propellant is free to move in the large empty volume, shooting upward with the propellant charge near the primer gives different velocity results than when shooting downward with the propellant charge forward. Finally, usage of subsonic ammunition, and its attending lower combustion pressures, frequently results in the inability to efficiently cycle semi-automatic or fully automatic weapons, such as the M16, M4, AR10, M2, M107s and the like. For repeating weapons to properly cycle, the propellant charge must produce sufficient gas pressure and/or volume to accelerate the projectile and to cycle the firing mechanism. Typical supersonic chamber pressures will be in the range from 30,000 psi to 70,000 psi. With a reduced quantity of propellant, subsonic ammunition generally fails to produce sufficient pressure to properly cycle the firing mechanism.
Over the years, a number of attempts have been made to safely and economically address these issues. These attempts have included the introduction of inert fillers, expandable inner sleeves that occupy the empty space between the propellant and the projectile (U.S. Pat. No. 4,157,684), insertion of flexible tubing (U.S. Pat. No. 6,283,035), foamed inserts (U.S. Pat. No. 5,770,815), stepped down stages in the discharge end of cartridge casings (U.S. Pat. No. 5,822,904), or complicated three and more component cartridges with rupturable walls and other complicated features (U.S. Pat. No. 4,958,567), all of which are incorporated herein by reference. Another approach has been to use standard cartridges in combination with non-standard propellants, such as is exemplified by U.S. Pat. Pub. No 2003/0131751, the disclosure of which is also incorporated herein by reference.
The result of such prior attempts to solve the production of reliable subsonic cartridges have been subsonic rounds that have a larger spread in velocity and thus less accuracy potential than what is desired. Moreover, associated production costs can be significantly greater then full velocity rounds because of the large number of additional manufacturing steps required to insert and secure the inserts used, or to construct the complicated shell casings required. Accordingly, a need exists to develop solutions that make it possible to manufacture better and more price competitive subsonic ammunition than previously available.